Cultural Practices to Reduce Pest and Disease in Avocado and Citrus

In many ways our pest and disease management of fruit tree crops are exacerbated by our cultural practices.  Avocado and citrus offer some very clear demonstrations of how we manage our trees can lead to reduced pesticide use.  From the beginning, our selection of rootstock and scion can help lessen pest and disease problems.  In both avocado and citrus we have good rootstocks which can handle problems, such as root rot more effectively than seedling rootstocks.  So it is imperative that if you know that drainage will be a problem, starting off with the right, healthy rootstock helps.  Also scion selection can have a major impact, as well.  For example, ‘Lamb’ avocado is much less prone to persea mite than is ‘Hass’.  This pest can significantly impact a spray program and planting ‘Lamb’ could mean virtually no sprays for this pest.  There are similar examples in citrus where one variety is more prone to a pest or disease than another.

Irrigation is probably the most important cultural factor in managing tree disease.  Over, under and improperly timed irrigations are the conditions necessary for many root diseases.  The Phytophthora spp. fungi are looking for distressed root systems brought on by waterlogging and other stressful situations.  Other conditions, such as wetted trunks can also bring on some trunk diseases, like gummosis in citrus and crown rot in avocado.  Simply preventing irrigation water on the trunks can limit these diseases.  Other diseases, such as black streak, stem blight and bacterial canker in avocado are bought on by soil moisture stress.

Nutrients, especially nitrogen management, has been long known to affect levels of insects, such as scale, mealy bug and aphid.  Encouraging lush growth helps sustain these insects, so reducing this growth tends to lower their numbers.  Managing when canopy growth occurs can affect pest severity.  Avocado thrips build their populations in the spring and moves easily from leaf to fruit causing significant scarring.  By promoting leaf growth at flowering time with a nitrogen application, keeps the insect on the leaves and reduces fruit scarring. This also promotes growth that replaces leaves that have been damaged by persea mite. Likewise the incidence of citrus leaf miner damage can be reduced if spring pruning is avoided so that a flush of growth does not occur at the same time as the population is building.  Timing of pruning is important in lemons to avoid wet periods of rain and fog to reduce the spread of hyphoderma wood rot fungus when its fruiting bodies are active.

Pruning can change pest pressure by changing the humidity in the canopy, introducing light and changing the climate supporting disease and pests.  By making spray coverage more thorough, it also makes for a more effective application.  Modified skirt pruning can have significant effects on mealy bug and scale control, fuller rose weevil incidence, ant colonization and snail damage.  It’s important that the trunk be protected as an avenue of movement for snail and ant control to get the best effects of this pruning. Skirt pruning also reduces problems with such weeds as bladder pod and the ladder effect of brown rot in citrus – fungal propagules splashed from the ground onto low-hanging fruit, which in turn is splashed to higher fruit. 

Keeping a canopy clean of dust and fire ash also makes for more efficient biological control.  Because predators are slowed in their search, they are less efficient.  They also spend more time grooming their sensory organs, and this also slows them down.  Parasites such as wasps are actually slowed by the physical abrasion to their tarsi.  Dust also creates a drier environment, which is more hospitable to our pest mites.  Watering picking rows, roads and even the trees themselves can lessen mite populations.  Use of cover crops can also reduce dust and potentially provide pollen and nectar for predators and parasites.  Of course cover crops create a whole new set of management issues, such as colder winter orchards and snails

Finally harvest timing to avoid pest and disease is often overlooked. In avocado, fruit is often set in clusters.  Greenhouse thrips love the microclimate created, and if in a size-pick the cluster is reduced, greenhouse thrips will often not be a problem.  Harvest timing is also important in citrus.  Fruit left too long on the tree can often develop septoria fungal spot.  Picking in a timely manner reduces the incidence of this disease.

These are just a few examples of how cultural practice at the right time can reduce pest and disease problems.

Dothiorella 2

Year Round IPM Guides

 

black streak

Black streak disease in avocado can be managed with irrigation as described in the IPM Year Round Program

Production quiz -- a challenge for growers

We’d like to challenge you to take the following quiz.  Take a minute to place a check mark next to all the practices you regularly employ in your operation.  Go ahead – we won’t be collecting them!

 

Part 1

Yes/ No I know what the nitrogen requirements (lbs actual N/acre/year or /tree/year) are for my crops

Yes/ No I know what the nitrogen levels are in soil amendments I use in my operation (compost, manure, crop residues, etc.)

Yes/ No I have lab analysis of my well/irrigation water.

Yes/ No I monitor tissue levels of nitrogen in my crops to help with fertilizer decisions.

Yes/ No I have put together a nutrient budget that considers all sources of nitrogen for the crops I produce.

 

Part 2

Yes/ No When I do apply nitrogen, applications are timed according to crop requirements.

Yes/ No I use fertigation to apply nitrogen.

Yes/ No Applications of nitrogen are split into smaller doses to improve efficiency of uptake.

Yes/ No I use cover crops that help manage nitrogen availability.

Yes/ No I manage irrigations to avoid nutrient loss below the rootzone of the crop.

 

If you marked yes to these as regular activities, you’ve just taken steps in showing how your production decisions can protect water quality.  The combined activities noted in Part 1 constitute a Management Practice that protects water quality by developing a nutrient budget to help apply only the appropriate amounts of fertilizer.  Activities in Part 2 may alone or in combination constitute Management Practices that help ensure fertilizers are applied efficiently.

Every grower uses ‘management practices’, many of which are meant to generate the best possible product for market.  Depending on who you’re talking with, the term ‘management practice’ can be something your Farm Advisor recommends (i.e., pruning to control tree height), your produce buyer suggests (protect avocados in bins from sun scald), or the term can have regulatory connotations.

You’ve all probably heard the term Best Management Practices.  Best Management Practice (BMP) is defined in the Federal Clean Water Act of 1987, as “a practice or combination of practices that is determined by a state to be the most effective means of preventing or reducing the amount of pollution generated by nonpoint sources to a level compatible with water quality goals.” The term “best” is subject to interpretation and point of view. In recognition of this, the Coastal Zone Reauthorization Amendment (2000) substituted the terms Management Measures and Management Practices.

How can you tell if any individual activity constitutes a Management Practice that meets the needs of a regulatory program to protect water quality?  Ask yourself this question:  Can the activity stand alone and result in water quality benefits?  Just knowing the nitrogen requirements of your crop doesn’t result in any water quality benefits – developing and using a nitrogen budget for your crop can.  A nitrogen budget that takes into account the nutrients applied in amendments, irrigation water, and fertilizers in meeting the requirements of your crop does have the potential to protect water quality from nitrogen pollution from your operation.

Some Management Practices can have water quality benefits as a stand alone activity.  Cover crops are recognized as a Management Practice that can help manage both sediment and nutrients to reduce the potential of pollution when used appropriately.

Water quality protection is being asked of all industries in California.  You have the opportunity to take credit for all of the activities you already do, like the ones listed above, that protect your local water bodies and/or groundwater from nonpoint source pollution from your operation.  Look for additional articles in the coming issues to help you in this effort.

For additional background information on water quality legislation, and nonpoint source pollution from agriculture you can download the following free publications from the University of California’s Farm Water Quality Program:

Water Pollution Control Legislation

 

Nonpoint Sources of Pollution from Irrigated Agriculture

Read more